1. Field of the Invention
This invention relates to a pulse shaping filter such as a spectroscopy filter to be used for radiation measurement, particularly for energy analysis of radioactive rays.
2. Description of the Related Art
A pulse-height analysis has been conventionally applied to energy analysis of radioactive rays in radiation measurement. The pulse shaping filter is used to obtain the optimum energy resolution for the analysis by improving the S/N ratio of the pulse. The pulse shaping filter mostly suited to the above purpose is a so-called cusp filter which generates a signal of cusp shape from a unit step signal.
This has been widely known from documents entitled "Realization of Optimum Pulse Shaping Filter" (pp. 43-46, theses of Symposium On Nuclear Electronics sponsored by EURATOM, 1969).
FIG. 1 is a structural block diagram of a conventional cusp filter. Referring to FIG. 1, a unit step signal A is inputted to a single integrating circuit 2 and a multiple integrating circuit 4 through an input terminal 1. An integrated signal B outputted from the single integrating circuit 2 is fed to a delay circuit 3 and a delayed signal C from the delay circuit 3 is subtracted by subtractor 5 from an integrated signal D outputted from the multiple integrating circuit 4. A subtracted signal E of the subtractor 5 is then sent to an amplifier 6, and an amplified signal from the amplifier is outputted from an output terminal 7.
The operation of the conventional cusp filter will be discussed below with reference to FIG. 2 showing the waveforms of the above signals A through E.
The single integrating circuit 2 generates the integrated signal B of an exponential shaping from the unit step signal A. On the other hand, the multiple integrating circuit 4 generates the integrated signal D of S shape from the unit step signal A. The integrated signal B is delayed by the delay circuit 3 to be the delayed signal C and when the delayed signal C is subtracted from the integrated signal D (D-C) by the subtractor 5, the subtracted signal E, in other words, the output signal of the cusp shaping filter is obtained.
It is to be noted here that the details of each function and the fact that the maximum S/N ratio is achieved by the function are disclosed in the aforementioned documents.
The conventional cusp filter improves the S/N ratio by approximately 1.02 of NF (noise figure) of the index generally employed in the radiation measurement field, which is considerably close to the theoretical limit 1.00. A semi-gaussian cusp filter which is most widely available among marketable cusp filters at present shows about 1.20 of NF.
Though the basic property of the conventional cusp filter having the above-described structure is excellent to be quite close to the theoretical limit, there exists the following problems since the final subtracted output of such a waveform as E illustrated in FIG. 2 after separately processing the input signal A in the single and multiple integrating circuits 2 and 4. That is because each pulse width of the delayed signal C and integrated signal D before subtraction is wide, which lowers the counting rate characteristic, and the saturation characteristic of each waveform of the signals C and D against the excessive signal A is different from each other, resulting in turbulence of the waveform after the subtraction.